1. Field of the Invention
The present invention relates to the process for preparing m,m'-dinitrobenzophenone by reacting benzophenone with nitric acid in oleum and isolating the m,m'-dinitrobenzophenone from the reaction product.
2. Description of the Prior Art
When benzophenone is reacted with nitric acid, a product can be obtained containing an isomeric mixture of dinitrobenzophenones, including o,o'-, o,m', m,m'-, m,p'- and p,p'-dinitrobenzophenones, which, in turn, can be subjected to hydrogenation to obtain an isomeric mixture of the corresponding diaminobenzophenones. The latter mixture can be reacted with 3,4,3',4'-benzophenone tetracarboxylic dianhydride (BTDA) to obtain a polyimide resin. Although the m,m'-, m,p'- and p,p'-diamino benzophenones in said latter mixture will react satisfactorily with BTDA to form desired long-chain polyimide resins, the ortho-diaminobenzophenones will react with BTDA to a far lesser extent, resulting in a mixture of long and relatively short polyimide resins. This is believed to be the result of hydrogen bonding between an ortho amine hydrogen and the carbonyl, which reduces the basicity of the compound and renders the compound less reactive with BTDA. It would be highly desirable, therefore, to reduce the content of ortho-substituted isomers of dinitrobenzophenones and substantially increase the content of m,m'-dinitrobenzophenone in a mixture containing the same.
In accordance with the teachings of U.S. Pat. No. 4,361,704 to Onopchenko, et al., benzophenone can be reacted with nitric acid to obtain a product containing an isomeric mixture of dinitrobenzophenones predominating in m,m'-dinitrobenzophenone, while substantially reducing the content of the isomers containing an ortho-nitro substituent with virtual disappearance of the o,o'-dinitrobenzophenone. In the Onopchenko, et al. process, benzophenone is reacted with nitric acid in oleum while maintaining critical weight ratios of benzophenone, sulfuric acid and sulfur trioxide in the feed.
Onopchenko, et al. teach in Column 2, lines 34 et. seq. that the dinitribenzophenones can be recovered from the reaction mixture by pouring the reaction mixture over ice and the resulting slurry can be subjected to filtration. The resulting filter cake however is a colored (i.e., tan) solid which requires a considerable amount of time and effort in continuous crushing, washing and refiltering to remove the impurities which are causing the coloration problem.